Many large mobile machines such as mining trucks, locomotives, marine applications and the like have recently begun using alternative fuels, alone or in conjunction with traditional fuels, to power their engines. For example, large displacement engines may use a gaseous fuel, alone or in combination with a traditional fuel such as diesel, to operate. Because of their relatively low densities, gaseous fuels, for example, natural gas or petroleum gas, are carried onboard vehicles in liquid form. These liquids, the most common including liquefied natural gas (LNG) or liquefied petroleum gas (LPG), are cryogenically stored in insulated tanks on the vehicles, from where a desired quantity of fuel is pumped, evaporated, and provided to fuel the engine.
The pumps that are typically used to deliver the LNG to the engine of the machine include pistons, which deliver the LNG to the engine. For example, while LNG may be stored at a pressure of about 300 psi, CNG for use by the engine may be provided at about 20.7 MPa. Such piston pumps, which are sometimes also referred to as cryogenic pumps, will often include a single piston that is reciprocally mounted in a cylinder bore. The piston is moved back and forth in the cylinder to draw in and then compress the gas. Power to move the piston may be provided by different means, the most common being electrical, mechanical or hydraulic power.
One example of a cryogenic pump can be found in U.S. Pat. No. 7,293,418 (the '418 patent), which describes a cryogenic, single-element pump for use in a vehicle. The pump discharges into an accumulator that is located within the tank, and uses a single piston pump that is connected to a drive section via a piston rod. The drive section is disposed outside of the tank.
Pumps such as the pump described in the '418 patent are required to generate sufficient gas pressure required to operate the engine, for example, sufficient pressure to overcome cylinder pressure in the case of a direct-injection gas engine application, but are not always up to the task. Under certain operating conditions, the engine will undergo relatively rapid transient changes in engine speed, engine load, and other engine operating conditions, which can be difficult for mechanical equipment (like fuel pumps) to keep up with. For example, when the engine operates at an idle condition, the LNG pump is operating slowly. When the engine operation changes such that the engine requires a high rate of fueling, such change can occur very quickly and the LNG pump may lag in providing a sufficient pressure. The pump lag will cause a pressure drop in the gas supply to the engine, because the engine will be consuming more than the pump can supply, at least temporarily. The lower gas pressure will often lead to lower fuel injection pressures in the engine cylinders, which can cause poor engine performance and emissions.